Large oil tanks may sit on the ground for thirty to fifty years or more. Due to various conditions and environmental factors, the bottom of the tank can become seriously corroded on its interior or top surface or on its back or ground-contacting surface. Where the corrosion gives rise to leakage of contents of the tank, evidently drastic environmental problems can occur.
Governmental regulations in place at present require regular inspections of the tank bottom to predict the remaining life thereof and to spot the locations where a proper repair or replacement is needed. Top surface conditions can be visually examined if the surface is not coated. But, the more prevalent back surface corrosion is both more serious and more difficult to detect and locate.
Two currently used inspection methods used to detect and locate back surface corrosion are based in the techniques of ultrasonics and flux leakage.
The ultrasonics method can be used to measure plate thickness and therefore detect plate thinning due to back surface corrosion. However, the technique is usually slow and tedious to apply in a sweeping inspection, due to the need for an oil or water based medium between the transducer and the plate under inspection. For a tank of one hundred and eighty feet in diameter, inspection time can run as much as five weeks for the completion of a track-mounted ultrasonic inspection. Further, the effectiveness of this practice is called into serious doubt if the tank bottom has its top surface sealed with a laminate or coating which has become delaminated or disbonded from the top surface. Thus, under such conditions, the ultrasonic pulse will not properly propagate into the plate, severely degrading the inspection capability. Accordingly, the ultrasonic method is generally more useful in determining the remaining plate thickness after an area of corrosion has previously been identified by the more easily and rapidly applied flux leakage method, which requires no liquid couplant and can inspect a tank of the noted size in about five days.
Extolling of the flux leakage method, however, cannot be such as to identify it as a problem-free practice. One of the major problems associated with the use of the flux leakage method is its inability to discern differences between top surface and back surface corrosion signal indications. Signals arising from top surface conditions can be greater in amplitude than back surface corrosion-indicating signals and the frequencies of the respective signals are virtually indiscernible.
Given the present state of flux leakage art, as above discussed, it has been almost impossible to this time to identify back surface corrosion as such in a significant percentage of tank bottoms while in the presence of extensive top surface corrosion.
From applicants' perspective, current flux leakage technology in plate inspection, and particularly in the inspection of tank bottoms, is lacking fundamentally in its inability to distinguish corrosion on one plate surface from that on an opposite plate surface, e.g., on a tank bottom top surface and on a tank bottom back surface. None of the known full magnetic saturation types of flux leakage systems, i.e., using either a coil type or a Hall element type leakage flux sensor, has the capacity for such distinction.